THE SEGMENTATION OF THE OVUM. 



101 



The protuberance soon becomes separated off from the larger part of 

 the egg as a small segment composed of clear protoplasm. From the larger 

 segment filled with food-yolk, a second small clear segment is next budded 

 off, and simultaneously (fig. 42, 2) the original small segment divides into 

 two. Thus there are formed four segments, one large and three small ; the 

 large segment as before being filled with food-yolk. The continuation 

 of a similar process of budding off and segmentation eventually results 

 in the formation of a considerable number of small and of one large 

 segment (fig. 42, 3). Between this large and the small segments is a seg- 

 mentation cavity. 



Eventually the large yolk segment, which has hitherto merely budded 

 off a series of small segments free from yolk, itself divides into two similar 

 parts. This process is then repeated (fig. 42, 4) and there is at last formed 

 a number of yolk segments filled with yolk spheres, which occupy the 

 place of the original large yolk segment. Between these yolk segments 

 and the small segments is placed the segmentation cavity. 



The segmentation of the ovum of Euaxes 1 resembles that of Unio in the 

 budding off of clear segments from those filled with yolk, but presents 

 many interesting individualities. 



A very peculiar modification of the ordinary Gasteropod segmentation is 

 that described by Bobretzky for Nassa mutabilis 2 . 



FIG. 43. SEGMENTATION OF NASSA MUTABILIS (from Bobretzky). A. Upper half 

 divided into two segments. B. One of these has fused with the large lower seg- 

 ment. C. Four small and one large segment, one of the former fusing with the 

 large segment. D. Each of the four segments has given rise to a small segment. 

 E. Small segments have increased to thirty-six. 



1 Kowalevsky, Mem. Akad. Petersburg, Series VII. 1871. 



2 Archiv.f. mikr. Anat. Vol. xili. 1877. 



